eli5: why do we know a note is off key even when we’ve never heard the piece of music before?

Question

Whenever I hear someone play an instrument, I can always tell when they’ve played the wrong note even if it’s my first time hearing it, why is that?

In: Other

Answer 1

Because not all ratios of frequencies sound good together.

Simple ratios generally sound the best, such as 2:1, 3:2, etc. Adding other frequencies generally sounds like “off key”.